Nucleoside analog reverse transcriptase inhibitors (NRTI) are integral components of therapy for HIV-1 infection. The currently approved NRTI have significant limitations that include short- and long-term toxicity, pharmacokinetic interactions with other antiretroviral drugs, and the selection of drug-resistant HIV-1 variants that are cross-resistant with other NRTI. Accordingly, there is a critical need to develop new NRTI that have excellent activity and safety profiles and exhibit little or no cross-resistance with existing drugs. We hypothesize that the rational design of NRTI, based on knowledge of both resistance mechanisms and structural components of NRTI that affect sensitivity to these resistance mechanisms, represents the best strategy to identify new NRTI that are active against drug resistant HIV-1. Using data derived from such structure-activity-resistance studies, we have identified a lead class of compounds, the 3'-azido-2',3'- dideoxypurines (ADPs), that retain potent activity against many drug-resistant variants of HIV-1. These include drug-resistant variants that (i) improve the ability of HIV-1 reverse transcriptase to discriminate between the natural dNTP substrate and the NRTI-triphosphate, and (ii) enhance the excision of the chain- terminating NRTI-monophosphate from the prematurely terminated DNA chain. In this application, we propose in-depth studies of structure-activity-resistance relationships, intracellular pharmacology, toxicity, and resistance selection to identify the most promising clinical candidates from the ADP class. This will be accomplished through three Specific Aims. In Aim 1, we will synthesize additional, novel ADP analogs, and prodrugs thereof, to optimize potency against both wild-type and NRTI-resistant HIV-1. In Aim 2, the ADPs that demonstrate favorable anti-HIV-1 activity and cross-resistance profiles will be further characterized by testing their potential toxicity toward human mitochondria and bone marrow progenitor cells. In Aim 3, we will select and characterize HIV-1 variants that are resistant to the ADPs, and elucidate the molecular mechanism(s) by which resistance is conferred. The identification and development of the novel ADPs through the rational design and assessments proposed in this application could help meet the expanding need for potent and safe NRTI that are active against drug-resistant HIV-1.